The present invention relates generally to radiant energy concentration and more particularly to high flux solar energy transformation systems suitable for use as energy sources for a variety of applications including laser pumping.
Numerous potential applications exist for concentrated solar photothermal energy. As one example, solar energy has been proposed as an energy source for the pumping of (e.g., satellite-based) neodymium-doped
YAG crystal lasers [see, e.g., Young, Applied Optics, 5(6), 993-997 (1966); Arashi et al., Japanese Journal of Applied Physics, 21(8), 1051-1053 (1984); and Weksler et al., "Solar Pumped Solid State Lasers" SPIE Proceedings, 736, (Jan. 15-16, 1987)] through use of focusing concentrators capable of "point" solar concentrations of about 10,000 suns at their focal plane. The laser pumping systems described have limited application because Nd:YAG is not a good absorber of solar energy. Solar energy application to the pumping of other laser sources, especially tunable lasers such as GSGG, alexandrite or Rhodamine 6G, has simply not been available, owing principally to the lack of systems capable of providing pumping thresholds on the order of 10,000 suns within the more limited absorption band (approximately 20 percent of the solar spectrum) within the broad band solar energy supplied. Indeed, prior attempts to secure extremely high solar energy flux intensification have completely failed to provide results even approaching factors of 50,000 suns or more needed to provide 10,000 suns in the relevant spectrum for pumping lasers of this type. It is noteworthy, for example, that the highest "peak" solar flux (within a small area at the center of the solar beam) reported to have been achieved was 1.6 kW cm.sup.-2 (which corresponds to a solar intensification of about 16,000 suns). When measured over an area capturing approximately 95 percent of the total energy, however, the average irradiance amount only to about 1300 suns. [See, Solar Thermal Test Facilities Users Association Newsletter, Apr. 30, 1980.]
Prior failures to achieve high flux concentrations are due to inherent properties of focusing (e.g., parabolic mirror) systems employed. A parabolic mirror forms an aplanatic image of the center of the sun in the center of its focal plane. It follows then from brightness conservation that the irradiance at the center of the image is given by EQU .sigma.T.sup.4 sin.sup.2 .phi.
where .sigma. is the Stefan-Boltzmann constant, T is the absolute temperature, and .phi. is the rim angle, i.e., the semi-angle subtended by the parabola from the center of the focal plane. According to this formula, one can achieve the n=1 thermodynamic limit of irradiance at the center of a 90.degree. rim angle paraboloid.
Off-axis aberrations spread the image, so that the irradiance decreases from the center. In fact, for the 90.degree. rim angle mirror the energy tails extend to infinity. A simple geometrical argument has been given to determine the minimum area required to collect 100 percent of the energy in the focal plane for any rim angle. The average geometric concentration ratio over this area is given by ##EQU1## This concentration ratio is a maximum for a 45.degree. rim angle, where it has a value of 11,500.
Apart from laser pumping applications directed to communication systems and potentially to radioisotype separation, extremely high solar flux concentration systems are conspicuously susceptible to use in procedures for determination of thermophysical properties (expansion, conductivity and the like) of various materials including high temperature ceramics, for the disposal of hazardous wastes by photothermally-induced reactions, for effecting high flux combustion, and for simulation of photothermal effects of nuclear explosion.
Of interest to the background of the invention are reports of developments by co-applicant Winston and his co-workers in the field of "non-imaging" optics applied to the transformation of radiant energy, including solar energy. See, e.g., W. T. Welford and R. Winston, The Optics of Non-Imaging Concentrators, (Academic Press, New York, 1978). Non-imaging concentrators are developed according to two basic design principles: the "extreme ray" or "maximum slope" principle (see, e.g., U.S. Pat. Nos. 3,923,381; 3,957,031; 4,002,499; 4,003,638; 4,045,246; 4,114,592; 4,130,107; 4,230,095; 4,240,692; and 4,483,007); and the "geometric vector flux" principle [see, e.g., U.S. Pat. No. 4,237,332; O'Gallagher et al., Solar Energy, 36(1), 37-44 (1986); Winston, SPIE Proceedings, 692, 224-226 (1986); and O'Gallagher et al., J. Opt. Soc. Am., 4, 66-68 (1987)]. Non-imaging concentrators may optionally be "filled" with a refractive, "dielectric" fluid or formed from a refractive solid, allowing for enhancement in concentrative capacity [see, e.g., U.S. Pat. No. 4,240,692; Ning et al., J. Opt. Soc. Am., 26, 300-305 (1987); and Ning et al., J. Opt. Soc. Am., 26, 1207-1212 (1987)].
Of particular interest to the background of the invention are proposals for development of composite, multi-stage systems involving both focusing and non-imaging devices for the concentration of radiant energy, including solar energy. See, e.g., Baranov, Applied Solar Energy, 2(3), 9-12 (1968); Ploke, Optik, 1, 31-43 (1967; and Winston et al., Applied Optics, 19, 347-351 (1980). Such composite devices, when employed in solar furnace and photovoltaic transformation applications, are generally designed to include relatively "fast" primary mirrors or lenses with focal ratios on the order of 0.5 to about 1.5 and non-imaging secondary concentrators with concentrative capacities on the order of 10 to 20.
There continues to exist a need in the art for novel systems capable of effecting high flux solar radiant energy transformation for use in a variety of photothermal energy application. Ideally, such systems would be capable of providing uniform concentration of solar flux with net solar intensification by factors in excess of those heretofore achieved.